1. Field of the Invention
This invention relates to energetic thermoplastic elastomers which are useful as binders of high energy compositions, such as propellants, especially rocket propellants and gun propellants, explosives munitions, gasifiers of vehicle supplemental restraint systems, or the like, and to methods for synthesizing the same.
2. Description of the Related Art
Solid high energy compositions, such as propellants, explosives, gas generants, and the like comprise solid particulates, such as fuel particulates and/or oxidizer particulates, dispersed and immobilized throughout a polymeric binder matrix.
Conventional solid composite propellant binders utilize cross-linked elastomers in which prepolymers are cross-linked by chemical curing agents. As outlined in detail in U.S. Pat. No. 4,361,526, there are important disadvantages to using cross-linked elastomers as binders. Cross-linked elastomers must be cast within a short period of time after addition of the curative, which time period is known as the “pot life.” Disposal of a cast, cross-linked propellant composition is difficult, and usually is accomplished by burning, which poses environmental problems. Furthermore, current state-of-the-art propellant compositions have serious problems that include their use of nonenergetic binders which have lower performance and high end-of-mix viscosities.
In view of the inherent disadvantages associated with the use of cross-linked elastomeric polymers as binder materials, there has been considerable interest in developing thermoplastic elastomers suitable as binders for solid, high energy compositions. However, many thermoplastic elastomers fail to meet important requirements expected of propellant formulations, particularly the requirement of being processible below about 120° C., it being desirable that a thermoplastic elastomeric polymer for use as a binder in a high energy system have a melting temperature of between about 60° C. and about 120° C. The melting temperature is desirably at least about 60° C. because the propellant composition may be subject to somewhat elevated temperatures during storage and transport, and significant softening of the propellant composition at such elevated temperatures is unwanted. The setting of the melting temperature at not more than 120° C. is determined by the instability, at elevated temperatures, of many components which ordinarily go into propellant compositions, particularly oxidizer particulates and energetic plasticizers. Many thermoplastic elastomers exhibit high melt viscosities which preclude high solids loading and many show considerable creep and/or shrinkage after processing. Cross-linkable thermoplastic elastomers typically obtain their thermoplastic properties from segments that form glassy domains which may contribute to physical properties adverse to their use as binders. Thermoplastic elastomers are block copolymers with the property of forming physical cross-links at predetermined temperatures. One thermoplastic elastomer, e.g., Kraton, brand TPE, obtains this property by having the glass transition point of one component block above room temperature. At temperatures below 109° C., the glassy blocks of Kraton form glassy domains and thus physically cross-link the amorphous segments. The strength of these elastomers depends upon the degree of phase separation. Thus, it remains desirable to have controlled, but significant, immiscibility between the two types of blocks, which is a function of their chemical structure and molecular weight. On the other hand, as the blocks become more immiscible, the melt viscosity increases, thus having a deleterious effect on the processibility of the material.
Above-mentioned U.S. Pat. No. 4,361,526 proposes a thermoplastic elastomeric binder which is a block copolymer of a diene and styrene, the styrene blocks providing a meltable crystal structure and the diene blocks imparting rubbery or elastomeric properties to the copolymer. The '526 patent states that this polymer is processed in a volatile organic solvent. Solvent processing is undesirable inasmuch as the dissolved composition cannot be cast in a conventional manner, e.g., into a rocket motor casing. Furthermore, solvent-based processing presents problems with respect to removal and recovery of solvent.
The preparation of energetic thermoplastic elastomers prepared from polyoxetane block copolymers has been proposed in U.S. Pat. No. 4,483,978 to Manser and U.S. Pat. No. 4,806,613 to Wardle (“the '613 patent”), the complete disclosures of which are incorporated herein by reference to the extent that these disclosures are compatible with this invention. According to the latter, these materials overcome the disadvantages associated with conventional cross-linked elastomers such as limited pot-life, high end-of-mix viscosity, and scrap disposal problems.
The thermoplastic materials proposed by the '613 patent involve elastomers having both (A) and (B) blocks, each derived from cyclic ethers, such as oxetane and oxetane derivatives and tetrahydrofuran (THF) and tetrahydrofuran derivatives. The monomer or combination of monomers of the (A) blocks are selected for providing a crystalline structure at usual ambient temperatures, such as below about 60° C., whereas the monomer or combination of monomers of the (B) blocks are selected to ensure an amorphous structure at usual ambient temperatures, such as above about −20° C. Typical of these materials is the random block copolymer (poly(3-azidomethyl-3-methyloxetane)-poly(3,3-bis(azidomethyl)oxetane), also known as poly(AMMO/BAMO).
Although the materials described by the '613 patent can be synthesized to exhibit acceptable mechanical properties for use as binders in explosive and propellant formulations, they have somewhat low densities. For example, poly(AMMO/BAMO) has a density of around 1.2 g/ml, with the precise value depending upon the proportions of A/B blocks. Additionally, the materials described by the '613 patent also have lower oxygen balance and energetic performance than other thermoset and thermoplastic energetic materials. The less than desirable oxygen balance and energy density of the thermoplastic elastomers disclosed by the '613 patent is a consequence of the use of polymer segments rich in unsymmetrical poly(3,3-disubstituted oxetane) or non-energetic polyether as the softer B block. The B block typically constitutes approximately 65–85% by weight of the thermoplastic elastomer.
It would, therefore, be a significant advancement in the art to provide energetic thermoplastic elastomer binders that are solid at room temperature and exhibit the excellent mechanical properties of the materials disclosed in the '613 patent, while possessing higher densities and improved oxygen balance and energetic performance.